1. Field of the Invention
The present invention relates to a light guide plate, a surface light source device and a display device. More specifically, the present invention relates to a light guide plate provided with a plurality of prisms on its upper surface and applicable to a liquid crystal display device, and a surface light source device and a display device respectively utilizing the same.
2. Description of the Prior Art
As a conventional such a kind of light guide plate and a surface light source device, there is an example applied to a liquid crystal display device 1 shown in FIG. 16, for example. As shown in FIG. 16, the liquid crystal display device 1 includes a reflective type liquid crystal panel unit 2, a light guide plate 3 provided on an upper surface of the liquid crystal panel unit 2 and a linear light source 4 extending in parallel with one side surface 3a of the light guide plate 3. A plurality of linear prisms 5 are formed parallel with the side surface 3a on an upper surface 3b of the light guide plate 3.
Referring to FIG. 17, a cross-section in a lengthwise direction of each of prisms 5, 5, . . . is formed in a mountain-shape, and a slope 7 opposed to the side surface 3a forms a reflecting surface 7. Specifically, a light irradiated into the light guide plate 3 from the linear light source 4 through the side surface 3a is reflected by each of reflecting surfaces 7,7, . . . and emitted from a lower surface 3c as shown in a dot-lined arrow 6. The emitted light is irradiated onto the liquid crystal panel unit 2, reflected upward by a reflecting plate (not shown) provided within the liquid crystal panel unit 2, and then, emitted outwardly from the upper surface 3b through the light guide plate 3 as shown in one dot-lined arrow 8 in FIG. 17.
It is noted that the further each of prisms 5, 5, . . . is formed from the linear light source 4, the higher the height H of the reflecting surface 7 is, i.e., the larger an area is made. Thus, even in a place far from the linear light source 4 (small in amount of emitted light to the reflecting surface 7), an amount of emitted light equal to that in a place near the linear light source 4 (large in amount of emitted light to the reflecting surface 7) can be obtained, and therefore, it is possible to obtain uniform brightness distribution throughout a lower surface 3c. 
An example of another prior art is disclosed in a Japanese patent Laying-open No. 2000-89225 [GO2F1/1335] laid-open on Mar. 31, 2000. This prior art is intended to prevent occurrence of moirxc3xa9 fringes due to interference by a prism and a pixel pattern of a liquid crystal display element by providing a predetermined angle between the lengthwise direction of the linear prism formed in plural on the light guide plate and a repetition direction of the pixel pattern of the liquid crystal display element.
It is probable that a combination of such the two prior arts can realize a light guide plate and a surface light source device having uniform brightness distribution and no moirxc3xa9 fringes.
However, only a combination of the above-described prior arts causes a following problem. That is, in a case a predetermined angle between a lengthwise direction of a plurality of prisms each of which has the reflecting surface different in height and a repetition direction of the pixel pattern of the liquid crystal display element (a direction vertical to the lengthwise direction of the linear light source) is formed, the height of each of prisms is different from each other along length direction of the linear light source. In such a case, even in the same distance from the linear light source, there occurs difference in amount of emitted light in the lengthwise direction of the linear light source, and therefore, there is a problem that uniform brightness distribution cannot be obtained.
Therefore, it is a primary object of the present invention to provide a novel light guide plate, and a surface light source device and a display device respectively utilizing the same.
Another object of the present invention is to provide a light guide plate capable of uniformalizing brightness distribution and preventing occurrence of moire fringes, and a surface light source device and a display device respectively utilizing the same.
The other object of the present invention is to provide a novel light guide plate capable of freely controlling brightness distribution, and a surface light source device and a display device respectively utilizing the same.
The light guide plate according to the present invention comprises a side surface to which a light is irradiated; and an upper surface formed with a plurality of lengthwise prisms extending in a direction oblique to the side surface, wherein a height of a plurality of slopes forming the plurality of prisms and opposed to the side surface is changed in a lengthwise direction of the prisms.
The light guide plate is formed with a plurality of prisms on its upper surface. Then, a height of slopes each of which forms prisms is changed in a lengthwise direction of the prisms. Accordingly, an amount of the light reflected by the slopes and emitted can be adjusted in a lengthwise direction of the prisms, and therefore, the brightness distribution can be freely controlled.
Consequently, a following surface light source device and a display device respectively utilizing such the light guide plate are also designed so as to control the brightness distribution freely and suitably.
A surface light source device comprises: a light guide plate including a side surface to which a light is irradiated and an upper surface formed with a plurality of lengthwise prisms extending in a direction oblique to the side surface, wherein a height of a plurality of slopes forming the plurality of prisms and opposed to the side surface is changed in a lengthwise direction of the prism; and a linear light source disposed extending in a direction parallel to the side surface.
A display device comprises: a light guide plate including a side surface to which a light is irradiated, an upper surface formed with a plurality of lengthwise prisms extending in a direction oblique to the side surface and a lower surface opposed to the upper surface, wherein a height of slopes forming the plurality of prisms and opposed to the side surface is changed in a lengthwise direction of the prisms; a linear light source disposed extending in a direction parallel to the side surface; and a liquid crystal display panel disposed on the lower surface.
Another light guide plate according to the present invention comprises: a side surface to which a light is irradiated; and an upper surface formed with a plurality of prisms extending in a direction oblique to the side surface, wherein a height of a plurality of slopes forming the plurality of prisms and opposed to the side surface is made higher away from the side surface.
In this light guide plate, a light irradiated from the side surface of the light guide plate is reflected by each reflecting surface forming each prism and opposed to the side surface. The height of the slope is higher away from the side surface, i.e., an incidence surface. Accordingly, even in a place far from the incidence surface, an amount of emitted light equal to that in the place nearer the incidence surface can be obtained. Therefore, although the prisms are formed in a direction oblique to the side surface to which a light is irradiated, since a height of the slopes of the prisms is changed according to the distance from the side surface, uniform brightness distribution can be obtained and occurrence of moirxc3xa9 fringes can be prevented.
Then, the same advantages can be expected in a following surface light source device and display device respectively utilizing the above-described light guide plate.
A surface light source device comprises: a light guide plate including a side surface to which a light is irradiated and an upper surface formed with a plurality of prisms extending in a direction oblique to the side surface, wherein a height of a plurality of slopes forming the plurality of prisms and opposed to the side surface is made higher away from the side surface; and a linear light source disposed extending in a direction parallel to the side surface.
A display device comprises: a light guide plate including a side surface to which a light is irradiated, an upper surface formed with a plurality of prisms extending in a direction oblique to the side surface and a lower surface opposed to the upper surface, wherein a height of a plurality of slopes forming the plurality of prisms and opposed to the side surface is made higher away from the side surface; a linear light source disposed extending in a direction parallel to the side surface; and a liquid crystal display panel disposed on the lower surface.
In either case, the prisms may be formed so as to come into contact with each other in a width direction. In this case, it is preferable that a cross-section of each prism in a lengthwise direction is formed in a mountain-shape.
Furthermore, the prisms may be formed to be spaced from each other in a width direction. In this case, it is preferable that a cross-section of each prism in a lengthwise direction is formed in an approximately V-shape.
The above described objects and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.